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A control apparatus for a brushless DC motor that rotatably drives the brushless DC motor including a rotor having a permanent magnet, and a stator having stator windings of a plurality of phases that generate a rotating magnetic field for rotating the rotor, by a current passage switching device co

A control apparatus for a brushless DC motor that rotatably drives the brushless DC motor including a rotor having a permanent magnet, and a stator having stator windings of a plurality of phases that generate a rotating magnetic field for rotating the rotor, by a current passage switching device constituted by a plurality of switching elements and performing successive commutation of current to the stator windings. The control apparatus includes: an angular error calculation device for calculating a sine value and a cosine value of an angular difference between an estimated rotation angle with respect to the rotation angle of the rotor and an actual rotation angle, based on a line voltage that is a difference between phase voltages of the plurality of phases on an input side of the stator winding and phase currents of the plurality of phases; and an observer for calculating the rotation angle of the rotor based on the sine value and the cosine value of the angular difference.

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What is claimed is: 1. A control apparatus for a brushless DC motor that rotatably drives the brushless DC motor including a rotor having a permanent magnet, and a stator having stator windings of a plurality of phases that generate a rotating magnetic field for rotating the rotor, by a current pas

What is claimed is: 1. A control apparatus for a brushless DC motor that rotatably drives the brushless DC motor including a rotor having a permanent magnet, and a stator having stator windings of a plurality of phases that generate a rotating magnetic field for rotating the rotor, by a current passage switching device constituted by a plurality of switching elements and performing successive commutation of current to the stator windings, the control apparatus comprising: an angular error calculation device for calculating a sine value and a cosine value of an angular difference between an estimated rotation angle with respect to the rotation angle of the rotor and an actual rotation angle, based on a line voltage that is a difference between phase voltages of the plurality of phases on an input side of the stator winding and phase currents of the plurality of phases; and an observer for calculating the rotation angle of the rotor based on the sine value and the cosine value of the angular difference. 2. The control apparatus for a brushless DC motor according to claim 1, wherein the angular error calculation device calculates a sine component and a cosine component of an induced voltage constituted by the sine value and the cosine value of the angular difference, and comprises: an angular velocity state function calculation device for calculating a state function in proportion to an angular velocity of the rotor based on the sine component and the cosine component of the induced voltage; and a normalization device for dividing the sine component of the induced voltage by the state function in proportion to the angular velocity of the rotor. 3. The control apparatus for a brushless DC motor according to claim 1, wherein the angular error calculation device calculates the sine component and the cosine component of the induced voltage constituted by the sine value and the cosine value of the angular difference, and comprises: a revolution speed measuring device for measuring the revolution speed of the brushless DC motor; an angular velocity calculation device for calculating the angular velocity of the rotor based on the revolution speed of the brushless DC motor; and a normalization device for dividing the sine component of the induced voltage by the angular velocity of the rotor. 4. The control apparatus for a brushless DC motor according to claim 1, wherein the angular error calculation device calculates the sine component and the cosine component of the induced voltage constituted by the sine value and the cosine value of the angular difference, and comprises: a coefficient acting device for causing a predetermined coefficient according to the estimated rotation angle to act on the sine component and the cosine component of the induced voltage; and a phase current differential value calculation device for calculating a differential value of the phase current, and the sine value and the cosine value of the angular difference is calculated based on Formula (1), where r is a phase resistance; V1, a first line voltage; V2; a second line voltage; I1, a first phase current; I2, a second phase current; l, a self inductance; m, a mutual inductance; θ^, an estimated rotation angle; θe, an angular difference between the estimated rotation angle and an actual rotation angle; ω, a rotational angular velocity of the rotor; Ke, an induced voltage constant; Vs, a sine component of the induced voltage; and Vc; a cosine component of the induced voltage. 5. The control apparatus for a brushless DC motor according to claim 4, wherein the phase current differential value calculation device calculates, by a least squares method, variations of current measured values per unit time at past predetermined times with respect to at least three of the current measured values of the phase currents that form time-series data, and comprises: a phase voltage correction device for correcting a time delay relating to the past predetermined times with respect to the phase voltages of the plurality of phases for calculating the line voltage; and a control angle correction device for correcting a time delay relating to an appropriate past time with respect to a control angle relating to the rotation angle of the rotor used when the phase currents of the plurality of phases are converted to a d-axis current and a q-axis current on d-q coordinates that form rotating orthogonal coordinates, and feedback control is performed so that a difference between a command value and a measured value of each current on the d-q coordinates becomes zero. 6. The control apparatus for a brushless DC motor according to claim 1, wherein the current passage switching device includes a bridge circuit with the plurality of switching elements connected by bridges, and the control apparatus further comprises a dead time correction device for correcting the phase voltages of the plurality of phases for calculating the line voltage, based on a dead time in which two of the switching elements connected in series for each phase in the bridge circuit are set to an off state, and polarities of the phase currents. 7. The control apparatus for a brushless DC motor according to claim 1, wherein the observer comprises an angular difference correction device for correcting the angular difference when the cosine value of the angular difference is negative. 8. The control apparatus for a brushless DC motor according to claim 1, wherein the observer comprises an angular difference correction device for correcting the angular difference according to a magnitude relation of absolute values of the sine value and the cosine value of the angular difference. 9. A control method for a brushless DC motor that rotatably drives the brushless DC motor including a rotor having a permanent magnet, and a stator having stator windings of a plurality of phases that generate a rotating magnetic field for rotating the rotor, the control method comprising the steps of: rotating the DC motor by performing successive commutation of current to the stator windings; calculating a sine value and a cosine value of an angular difference between an estimated rotation angle with respect to the rotation angle of the rotor and an actual rotation angle, based on a line voltage that is a difference between phase voltages of the plurality of phases on an input side of the stator winding and phase currents of the plurality of phases; and calculating the rotation angle of the rotor based on the sine value and the cosine value of the angular difference. 10. The control method for a brushless DC motor according to claim 9, further comprising the steps of: calculating a sine component and a cosine component of an induced voltage constituted by the sine value and the cosine value of the angular difference; calculating a state function in proportion to an angular velocity of the rotor based on the sine component and the cosine component of the induced voltage; and dividing the sine component of the induced voltage by the state function in proportion to the angular velocity of the rotor so as to be normalized. 11. The control method for a brushless DC motor according to claim 9, further comprising the steps of: calculating a sine component and a cosine component of an induced voltage constituted by the sine value and the cosine value of the angular difference; measuring the revolution speed of the brushless DC motor; calculating the angular velocity of the rotor based on the revolution speed of the brushless DC motor; and dividing the sine component of the induced voltage by the state function in proportion to the angular velocity of the rotor so as to be normalized.